3.4 Protein Synthesis

DNA's genetic code is the blueprint for life. It contains instructions for making proteins, the workhorses of our cells. This code is read and translated through a complex process involving transcription and translation, turning DNA's language into functional molecules.

Ribosomes play a starring role in this protein-making process. These cellular machines read the genetic instructions and assemble amino acids into proteins. It's like a molecular assembly line, churning out the building blocks that keep our bodies running.

Genetic Code and Protein Synthesis

DNA code for protein structure

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• DNA contains genes which are nucleotide sequences coding for specific proteins
  • Genes composed of exons (coding regions) and introns (non-coding regions)
• Genetic code is set of rules determining how nucleotide sequence in DNA translates into amino acid sequence in protein
  • Genetic code based on codons which are nucleotide triplets
  • Each codon specifies particular amino acid (Methionine, Proline) or stop signal (UAA, UAG, UGA)
• Codon sequence in gene determines amino acid sequence in resulting protein
  • Amino acid order in protein determines its primary structure
  • Protein's primary structure influences folding and final 3D shape which determines function (enzymes, structural proteins)

Components of transcription process

• Transcription is process of synthesizing RNA from DNA template
  • Occurs in nucleus of eukaryotic cells (human, plant)
• Key steps of transcription: initiation, elongation, termination
  1. Initiation: RNA polymerase binds to promoter region on DNA and separates DNA strands
  2. Elongation: RNA polymerase moves along DNA template strand and synthesizes complementary RNA strand
  3. Termination: RNA polymerase reaches terminator sequence and releases newly synthesized RNA and DNA template
• Main components involved in transcription:
  • DNA template strand provides genetic information
  • RNA polymerase enzyme catalyzes RNA synthesis
  • Ribonucleoside triphosphates (ATP, GTP, CTP, UTP) serve as building blocks for RNA (nucleotides)
• Resulting product of transcription is pre-mRNA molecule which undergoes further processing to become mature mRNA (splicing, capping, polyadenylation)

Translation and RNA roles

• Translation is process of synthesizing protein from mRNA template
  • Occurs in cytoplasm of eukaryotic cells on ribosomes (rough ER)
• Translation stages: initiation, elongation, termination
  1. Initiation: Small ribosomal subunit binds to start codon (AUG) on mRNA with help of initiation factors and special initiator tRNA
  2. Elongation: Large ribosomal subunit joins complex, tRNA molecules bring amino acids to ribosome which are linked to form growing polypeptide chain
  3. Termination: Ribosome reaches stop codon (UAA, UAG, UGA), release factors cause ribosome to release completed polypeptide chain and dissociate from mRNA
• tRNA (transfer RNA) molecules crucial in translation
  • Each tRNA has anticodon complementary to specific codon on mRNA
  • tRNAs carry corresponding amino acid matching codon (tRNA-Met carries Methionine)
  • tRNAs bring appropriate amino acids to ribosome based on mRNA codons
• mRNA (messenger RNA) is template for protein synthesis
  • Carries genetic information from DNA to ribosomes
  • Codon sequence in mRNA determines amino acid order in resulting protein

Ribosomes in protein synthesis

• Ribosomes are sites of protein synthesis in cells
  • Composed of ###ribosomal_RNA_(rRNA)_0### and proteins
  • Eukaryotic ribosomes have large (60S) and small (40S) subunits
• Ribosomes facilitate protein synthesis by providing platform for amino acid assembly into polypeptide chains
  • Ribosome has three sites: A (aminoacyl), P (peptidyl), E (exit)
    • A site binds incoming tRNA carrying next amino acid
    • P site holds tRNA carrying growing polypeptide chain
    • E site holds empty tRNA before it dissociates from ribosome
• Ribosomes catalyze peptide bond formation between amino acids
  • Peptidyl transferase center in large ribosomal subunit catalyzes peptide bond formation between amino acids in A and P sites
• Ribosomes move along mRNA 5' to 3', reading codons and adding amino acids to growing polypeptide chain
  • Process continues until ribosome reaches stop codon, completed polypeptide chain released (insulin, collagen)

From Gene to Functional Protein

• Central dogma of molecular biology describes flow of genetic information: DNA → RNA → Protein
• Amino acids are building blocks of proteins, joined together to form polypeptide chains during translation
• After translation, proteins undergo post-translational modifications to achieve final functional form
• Protein folding occurs as newly synthesized polypeptide chain adopts its three-dimensional structure
  • Folding is crucial for protein function and is influenced by amino acid sequence and cellular environment